This invention relates to methods for producing metallic patterns upon substrates. The invention is especially advantageous in applications requiring dimensional control and positional stability of a conductive metallic pattern upon a transparent substrate. The pattern so produced might simply remain on one side of a transparent substrate, as in some electric defrosters for automotive windows. Alternatively, it might be sandwiched between two substrates subsequently laminated together, as in some radio antennae imbedded within automotive windshields. A further example is a digitizer platen suitable for rear projection of images to be digitized. The dimensional accuracy and relative position of the conductive grids within such a digitizer platen must be carefully controlled if the digitizer is to be accurate. Furthermore, the method of producing the grids must not interfere with the transparency of the platen.
There exist techniques for vapor deposition of a metallic film upon a substrate. These techniques suffer from a number of disadvantages: high cost; low yield; poor adhesion of the metal to certain substrates, such as glass; lack of uniform thickness of the deposited metal; and, lack of solderability to the resulting pattern. Sputtering techniques suffer similar disadvantages. In addition, both these techniques suffer from the additional disadvantage that only relatively thin conductors can be reliably produced, rendering the conductors unsuitable for use in high current applications.
There are techniques for attaching a metal foil to a substrate with a resin. However, after the foil is etched to produce the desired pattern the resin itself remains. The resin interferes substantially with the transparency of transparent substrates, and its continued presence may be unwanted even with nontransparent substrates. The resin is extremely difficult to remove since solvents that remove the resin also loosen the foil.
There are also techniques for simply placing a pre-formed metallic foil, or a pattern wound from wire, between two transparent substrates before they are laminated together with a transparent adhesive. The main disadvantage with this technique is that the adhesive material used to laminate the substrates may flow substantially during the lamination process, and may carry the pattern with it and prevent the pattern from having precise dimensional and positional characteristics.
Because of these difficulties, transparent digitizer platens have heretofore either been difficult to produce or of less than optimum transparency.
Accordingly, a primary object of the invention is to provide a method for accurately producing a metallic pattern of good dimensional stability upon a transparent substrate without interfering with the transparency of the substrate in those regions where the metallic pattern is absent.
A further object of the invention is to provide a method for producing upon a transparent substrate a solderable metallic pattern having good adhesion to the substrate, without interfering with the transparency of the substrate in those regions where the metallic pattern is absent.
Another object of the invention is to provide a method for producing upon a transparent substrate a metallic pattern having electrical properties that are uniformly controllable without interfering with the transparency of the substrate in those regions where the metallic pattern is absent.
A further object of the invention is to provide a method for producing highly accurate metallic grids sandwiched between plates of transparent material, for use as a platen in a digitizer.
A still further object of the invention is to provide a method for attaching a metal foil to a substrate such that no undesirable residue remains after portions of the foil are etched away.
These and other objects of the invention will become apparent to those skilled in the art as the summary and detailed description of the invention proceed.
A summary of a preferred method of practicing the invention is as follows. First, a transparent substrate upon which a metallic pattern is to be produced is thoroughly cleaned. The substrate then has laminated to it a layer of photopolymerizable adhesive. Next, a layer of metal foil is laminated to the layer of photopolymerizable adhesive. A layer of photosensitive resist is then applied to the exterior side of the metal foil. The layer of photosensitive resist is exposed according to the desired pattern and the unwanted metal in the foil is uncovered by removal of the corresponding portions of the photosensitive resist. This is done without exposing the layer of photopolymerizable adhesive. Next, the unwanted metal is etched away, uncovering portions of the layer of unexposed photopolymerizable adhesive.
Finally, the method includes removal of the uncovered portions of the photopolymerizable adhesive. Since the adhesive layer is still unexposed it is still removable by etching in a suitable solution. The metallic pattern itself serves as a resist in the removal of the uncovered portions of the adhesive layer. The unremoved adhesive is hardened by at last exposing the remaining portions of the adhesive layer. This is easily done by using the transparency of the substrate to illuminate the adhesive with actinic radiation from the unlaminated side of the substrate. This polymerizes the adhesive and renders it hard.
In the case where a digitizer platen is being constructed, two such metallic patterns, each in the form of a grid, are produced. One is produced upon the underside of an upper piece of glass, and the other is produced on the upper side of a lower piece of glass. Ribbon conductors are soldered to the grids for connecting them to the circuitry of the digitizer. Finally, the two pieces of glass are registered and laminated together with an intermediate layer of polyvinyl butyral resin to form a single piece of safety glass. The resin not only causes adhesion between the two pieces of glass, but also insulates the upper and lower metallic grids from each other.